Selective ion source for high intensity focused and collimated ion beams - coupling with high resolution cycloidal path sector

ABSTRACT

A mass spectrometer is described in which the ions are submitted to the action of a uniform adjustable electrical field {right arrow over (E)} 1 , within a set of plane parallel electrodes a 1 , . . . , a i,  . . . a n  fitted with properly located slits for the ions transmission, and to the action of a uniform magnetic induction {right arrow over (B)} 1 . A reference system x,y is considered in a plane perpendicular to {right arrow over (B)} 1 , the axis x and y being respectively perpendicular and parallel to {right arrow over (E)} 1 , and the origin of the reference system being fixed at an average starting point of the ions. The crossed fields {right arrow over (E)} 1 ,{right arrow over (B)} 1 , act together in an area where y&lt;d and the magnetic induction {right arrow over (B)} 1  acts alone in a further area where y&gt;d, d being a distance separating the average starting point of the ions from the electrode α n . The selection slit S 1  is located at coordinates x=2.1d and y=2d, and the value of E 1  that is applied for the selection of the ions having the number of mass n is defined by  
           E   1     =         2      d     n          e   m          B   1   2         ,                 
 
     with  
       e   m                 
 
     with e/m corresponding to a ratio charge/mass for H + . The ions are created by electronic bombardment only in the vicinity of a plane parallel to {right arrow over (B)} 1 , making an angle of 45° with {right arrow over (E)} 1 , and a heated filament F for emitting electrons being stretched above the electrodes a 1 ,a 2 , such that a resulting selected ion beam is parallel to the x axis when crossing S 1 . The ion beam may feed, under optimal conditions, a “cycloid path” mass spectrometer or a 90° magnetic sector. The system provides improved sensitivity and a high resolution within a small and very simple instrument.

BACKGROUND OF THE INVENTION

[0001] Ions sources known from prior art, as described for example inthe publication “Electron Optics” by Pierre Grivet, are subject to anumber of limitations.

[0002] In order to minimize the energy dispersion of ions generated inthe ions source, the ionisation space is reduced.

[0003] As a result, ionisation cells may be nearly closed and the ionsextraction has a rather poor yield.

[0004] In case that it is required to improve the collimating, this maybe achieved by a selection through a number of narrow slits.

[0005] However introducing selection and reducing the ionisation spacemay drastically reduce the sensitivity to a value in the order of 10⁻⁴A/Torr or less, with the exception of very big instruments.

[0006] The applicant has earlier described a high intensity selectiveion source in a French patent application Nr 0009081 filed 17 Jul. 2000,in which ions created by electronic bombardment in a large volume andhaving the same number of mass n, can be focused on a narrow slit S₁ inorder to obtain an emerging ion beam

SUMMARY OF THE INVENTION

[0007] In a first aspect the invention provides a mass spectrometer inwhich the ions are submitted to the action of a uniform adjustableelectrical field E₁, within a set of plane parallel electrodes a₁, . . ., a_(i,) . . . a_(n) fitted with properly located slits for the ionstransmission, and to the action of a uniform magnetic induction {rightarrow over (B)}₁. A reference system x,y is considered in a planeperpendicular to {right arrow over (B)}₁, the axis x and y beingrespectively perpendicular and parallel to {right arrow over (E)}₁, andthe origin of the reference system being fixed at an average startingpoint of the ions. The crossed fields {right arrow over (E)}₁,{rightarrow over (B)}₁, act together in an area where y<d and the magneticinduction {right arrow over (B)}₁ acts alone in a further area wherey>d, d being a distance separating the average starting point of theions from the electrode a_(n). The selection slit S₁ is located atcoordinates x=2.1d and y=2d, and the value of E₁ that is applied for theselection of the ions having the number of mass n is defined by E₁=${E_{1} = {\frac{2d}{n}\frac{e}{m}B_{1}^{2}}},{{with}\quad \frac{e}{m}}$

[0008] corresponding to a ratio charge/mass for H⁺. The ions are createdby electronic bombardment only in the vicinity of a plane parallel to{right arrow over (B)}₁, making an angle of 45° with {right arrow over(E)}₁, and a heated filament F for emitting electrons is stretched abovethe electrodes a₁,a₂, along a line where x=y, the electron beam beinglimited by a flat rectangular diaphragm parallel to F and locatedbetween F and the electrodes a₁,a₂, such that a resulting selected ionbeam is parallel to the x axis when crossing S₁.

[0009] In a preferred embodiment of the invention, the ions aresubmitted to a second couple of crossed fields {right arrow over(E)}₂,{right arrow over (B)}₁, at the exit S₁, {right arrow over (E)}₂being created within a second set of plane parallel electrodes b₁, . . ., b_(i), . . . b_(n), also fitted with properly located slits for theions transmission. A value of the electric field E₂ is equal to E₁ cos45° and a direction of the electric field E₂ makes an angle of 45° withE₁. A second selection slit S₂ is located on the cycloid path on theelectrode b_(n) at a point defined by coordinates X≅8.9d and Y=0 in afurther reference system that is defined by axis X and Y, wherein the Xaxis and the Y axis are respectively perpendicular and parallel to E₂.An origin of the further reference system is fixed at S₁.

BRIEF DESCRIPTION OF THE FIGURES

[0010] The invention will now be described in greater detail withreference to the accompanying drawings, in which

[0011]FIG. 1 contains an illustration of an embodiment of a massspectrometer in accordance with the invention,

[0012]FIG. 2 contains an illustration of an embodiment of a massspectrometer in accordance with the invention,

[0013]FIG. 3 contains an illustration of an embodiment of a massspectrometer in accordance with the invention.

EXAMPLES OF PREFERRED EMBODIMENTS

[0014] We will now describe examples of systems that bring importantimprovements to the systems known from prior art. One advantage of thedescribed examples is that they allow the emerging ion beam to be wellcollimated. The collimated emerging ion beam provides a powerful ionsource that can then feed, in optimal conditions, a “cycloid path” massspectrometer or a 90° magnetic sector fitted with a very narrow gap.

[0015] Referring to FIG. 1, ions created by electronic bombardment in anions source 1 are submitted to the action of a uniform adjustableelectrical field {right arrow over (E)}₁, between a set of planeparallel electrodes a₁, a₂, . . . , a_(n) fitted with properly locatedslits for the transmission of the ions.

[0016] The ions are also submitted to the action of a fixed uniformmagnetic induction {right arrow over (B)}₁, that is perpendicular to{right arrow over (E)}₁, created by an external magnetic circuitsandwiching a box of the instrument (not shown in FIG. 1).

[0017] We will use a reference system defined by two axis x,y in a planeperpendicular to {right arrow over (B)}₁, the axis x and y beingrespectively perpendicular and parallel to {right arrow over (E)}₁, andan origin of the reference system being fixed at a determined averagestarting point 2 of the ions source 1.

[0018] The electrical field {right arrow over (E)}₁ and the magneticinduction {right arrow over (B)}₁ act in a first area where the value ofy verifies

y<d

[0019] d being a distance separating the determined average startingpoint 2 from the electrode a_(n).

[0020] The magnetic induction {right arrow over (B)}₁ acts alone in asecond area where the value of y verifies

y>d.

[0021] The ions follow a cycloid path in the first area (y<d) and acircular path with a radius of curvature R in the second area (y>d).

[0022] For the ions having a number of mass n and for${{\overset{\rightarrow}{E}}_{1} = {\frac{2d\quad \omega \quad B}{n}\left( {{{{in}\quad {which}\quad \omega} = {\frac{e}{m}B}},{{with}\quad \frac{e}{m}\quad {being}\quad {the}{\quad \quad}{ratio}\quad {of}\quad {charge}\quad {to}{\quad \quad}{mass}\quad {for}\quad {the}\quad {hydrogen}\quad {ion}\quad H^{+}}} \right)}},$

[0023] being the ratio of charge to mass for the hydrogen ion H⁺), theradius of curvature R is:

R=2d

[0024] The centre of curvature 3 having coordinates (x₀,y₀) in thereference system is located at a point defined as follows:

x ₀=2.1d=1.05R, and

y ₀=0.

[0025] A selection slit S₁, is located at a point having followingcoordinates:

x=2.1d=1.05R, and

y=2d=R.

[0026] For the proper determined value of {right arrow over (E)}₁, theions n cross the selection slit S₁, the trajectories of the ions beingat this point parallel to the x axis.

[0027] Let us consider now the case where the starting point for theions varies from (0.0) to (Δx,Δy), Δx and Δy being relatively small inregard of R.

[0028] Varying Δx only will just cause a simple translation Δx of thetrajectory. The new centre of curvature having coordinates(x_(Δx),y_(Δx)) will be located at following coordinates:

x _(Δx)=1.05R+Δx, and

y _(Δx)=0.

[0029] The ions are still crossing S₁, but their trajectories make atthis point an angle α=Δx/R with the x axis.

[0030] Varying Δy only, modifies the interval d where {right arrow over(E)}₁ is acting, and of course the ions speed and the radius ofcurvature of the trajectories. We have:$\frac{R + {\Delta \quad R}}{R} = {\left( \frac{d - {\Delta \quad y}}{d} \right)^{\frac{1}{2}} = {\left. \left( \frac{R - {2\Delta \quad y}}{R} \right)^{\frac{1}{2}}\rightarrow{\Delta \quad R} \right. = {{- \Delta}\quad y}}}$

[0031] Accordingly the new centre of curvature having coordinates(x_(Δy),y_(Δy)) is located as follows:

x _(Δy)=1.05R−Δy and

y _(Δy) =Δy.

[0032] The ions n are still crossing S₁ but at this level, thetrajectories are making an angle$\alpha = {- \frac{\Delta \quad y}{R}}$

[0033] with the x axis.

[0034] So if we take in account both a variation of Δx and Δy, thecentre of curvature having coordinates (x_(ΔxΔy), y_(ΔxΔy)) is locatedas follows:

x _(ΔxΔy)=1.05R+Δx−Δy, and

y _(ΔxΔy) =Δy,

[0035] the radius of curvature being equal to R−Δy.

[0036] The trajectories at the level S₁ make an angle$\alpha = \frac{{\Delta \quad x}\quad - {\Delta \quad y}}{R}$

[0037] with the x axis.

[0038] IMPORTANT PARTICULAR CASE: Δx=Δy and α=o.

[0039] This case corresponds to the ions created in the vicinity of aplane parallel to {right arrow over (B)}₁ intersecting the plane xyfollowing a line x=y . All of these ions having the number of mass n,crossing S₁ for${E_{1} = {\frac{2d\quad \omega \quad B_{1}}{n} = \frac{R\quad \omega \quad B_{1}}{n}}},$

[0040] will have their trajectories at the level S₁ perfectly parallelto the x axis.

[0041] A setting allowing the creation of only these ions and so, toobtain at the exit S₁ a perfectly collimated beam, can be very simple.

[0042] Referring to FIG. 2, a heated filament F, emitter of the ionisingelectrons, is stretched above the electrodes α₁,α2, following a linex=y. The electron beam generated by the heated filament F is limited bya flat small electrode C, fitted with a narrow rectangular diaphragm,parallel to F and located between F and α₁,α₂.

[0043] We will now describe the introduction of the collimated ion beamin a “cycloid path” sector, where the ions will be submitted to theaction of a second couple of crossed fields {right arrow over(E)}₂,{right arrow over (B)}₁.

[0044] The field {right arrow over (E)}₂ will be equal to {right arrowover (E)}₁ cos 45° and will make an angle of 45° with {right arrow over(E)}₁ and with the ion beam at S₁. We will use for this sector a secondreference system X, Y in the same plane perpendicular to {right arrowover (B)}₁. The X- and Y-axis are respectively perpendicular andparallel to {right arrow over (E)}₂ and the origin is fixed at S₁.

[0045] {right arrow over (E)}₂ is established within a set of planeparallel electrodes b₁, . . . b_(i) . . . , b_(n).

[0046] The initial speed v of the ions is given by:$v = {\left( \frac{2{eE}_{1}d}{nm} \right)^{\frac{1}{2}} = {\frac{R\quad \omega}{n} = \frac{E_{1}}{B_{1}}}}$

[0047] So, the components X′_(o) and Y′_(o) of v are given by$X_{0}^{\prime} = {Y_{0}^{\prime} = {{\frac{E_{1}}{B_{1}}\cos \quad 45^{{^\circ}}} = {\frac{E_{2}}{B_{1}}.}}}$

[0048] Due to the different starting points of the ions in the source,these values are only an average but ΔX′_(o) is always equal to ΔY′_(o).

[0049] The equations of the trajectories are, in general:$X = {{\frac{n}{\omega}{Y_{0}^{\prime}\left( {1 - {\cos \frac{\omega \quad t}{n}}} \right)}} - {\frac{n}{\omega}\left( {{- X_{0}^{\prime}} + \frac{E_{2}}{B_{1}}} \right)\sin \frac{\omega \quad t}{n}} + {\frac{n}{\omega}\frac{E_{2}}{B_{1}}\frac{\omega \quad t}{n}}}$$Y = {{\frac{n}{\omega}{Y_{0}^{\prime}\left( {\sin \frac{\omega \quad t}{n}} \right)}} + {\frac{n}{\omega}\left( {{- X_{0}^{\prime}} + \frac{E_{2}}{B_{1}}} \right)\left( {1 - {\cos \frac{\omega \quad t}{n}}} \right)}}$

[0050] The conditions of unicity of the trajectories for smallvariations of X′_(o) and Y′_(o) are:$\frac{\frac{\delta \quad Y}{\delta \quad X_{0}^{\prime}}}{\frac{\delta \quad X}{\delta \quad X_{0}^{\prime}}} = {\frac{\frac{\delta \quad Y}{\delta \quad Y_{0}^{\prime}}}{\frac{\delta \quad X}{\delta \quad Y_{0}^{\prime}}} = \frac{\frac{\delta \quad Y}{\delta \quad t}}{\frac{\delta \quad X}{\delta \quad t}}}$

[0051] It is easy to check that, for$X_{o}^{\prime} = {Y_{o}^{\prime} = \frac{E_{2}}{B_{1}}}$

[0052] and ΔX′_(o)=ΔY′_(o) these conditions are always fulfilled. Allthe ions n of the collimated beam at the exit of S₁ will follow exactlythe same path, even when having different initial energies.

[0053] Of course, for the same value of E₂, the ions having a number ofmass ≠n, i.e. having a value different from n, will follow differentpaths and be discarded.

[0054] We did not take in account the adverse effect of the randominitial energy of the ions when created in the source. This energy, for“fragment ions” can be of the order of 1 eV.

[0055] The trajectories of those ions are not collimated at the exit S₁but converge towards the ideal trajectory. Detailed computations showthat they will cross it at a point of coordinates X≅1.5d and Y≅d.

[0056] In any case, it is well known that in a “cycloid path” massspectrometer, the ions are perfectly focused, after a flying time${t = \frac{2\quad \Pi \quad n}{\omega}},$

[0057] on a line having, in our case, the coordinates X=8.9d and Y=O.This final decision slit S₂ is, of course, located there.

[0058] A different coupling can be done easily with a classical 90°magnetic sector. This is shown in the example illustrated in FIG. 3. Theion beam at the exit S₁ of the ion source has the geometry of a flatplanar ribbon that can be introduced in the narrow gap of a magneticcircuit M₂.

[0059] Due to the small gap, the magnetic induction {right arrow over(B)}₂ can be much larger than {right arrow over (B)}₁ in the firstmagnetic circuit M₁. {right arrow over (B)}₂ is perpendicular to theplane of the ion beam and to {right arrow over (B)}₁, and so are the twomagnetic circuits M₁, M₂.

[0060] In the gap of M₂, the ions n will follow circular trajectorieshaving a radius $R_{2} = {2d\quad {\frac{B_{1}}{B_{2}}.}}$

[0061] As B₂>>B₁, R₂ is much smaller than R₁. In order to increase R₂(and the resolution) it is easy to increase the ions energy with anaccelerating electrical field E₂=kE₁, between two electrodes F₁ and F₂separated by a distance pd and located between the to magnetic circuitsM₁,M₂. The initial energy of the ions having the mass number n whenentering the gap in M₂ will be then equal to

eE ₁ d(1+kp)

[0062] The radius of curvature R₂ of their paths will be, allcomputations done,$R_{2} = {2d\frac{B_{1}}{B_{2}}\left( {1 + {kp}} \right)^{\frac{1}{2}}}$

[0063] A final selection hole S₂ is located at X=Y=R₂ where the ion beamsection is reduced to a point.

[0064] Numerical Application

[0065] Let us choose B₂=3B₁ R₂=R₁=2d p=0.5.

[0066] In this case the value of k is as follows: k=16. It is easy tocheck that, with these values, the aberrations at S₂ are negligible.

[0067] Behind S₂, a Faraday cup or an internal amplifier C (achanneltron for instance) will receive the selected ion beam. Thesensibility is always limited by the grossly continuous noise of theamplifier. But, if the ion beam is modulated by a grid α, located forinstance at S₁ and polarised at an alternative potential with a fixedfrequency, the useful signal can be detected and amplified independentlyof the noise. The ratio signal to noise can be greatly improved.

CONCLUSION

[0068] This very simple system has both high sensitivity and highresolution.

[0069] The high sensitivity is mainly due to the fact that theionisation volume is large and to the fact that the Open geometry of thesystem allows a complete extraction of the ions.

[0070] The high resolution is mainly due to the fact that the secondselection sector is fed by a preselected collimated ion beam.

[0071] While the invention has been described with respect to a limitednumber of embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A mass spectrometer in which the ions are submitted to the action ofa uniform adjustable electrical field {right arrow over (E)}₁, within aset of plane parallel electrodes a₁, . . . , a_(i,) . . . a_(n) fittedwith properly located slits for the ions transmission, and to the actionof a uniform magnetic induction {right arrow over (B)}₁, wherein areference system x,y is considered in a plane perpendicular to {rightarrow over (B)}₁, the axis x and y being respectively perpendicular andparallel to {right arrow over (E)}₁, and the origin of the referencesystem being fixed at an average starting point of the ions, the crossedfields {right arrow over (E)}₁,{right arrow over (B)}₁ acting togetherin an area where y<d and the magnetic induction {right arrow over (B)}₁acting alone in a further area where y>d, d being a distance separatingthe average starting point of the ions from the electrode a_(n), theselection slit S₁ being located at coordinates x=2.1d and y=2d, and thevalue of E₁ that is applied for the selection of the ions having thenumber of mass n being defined by${{E_{1} = {\frac{2d}{n}\frac{e}{m}B_{1}^{2}}},{{with}\quad \frac{e}{m}}}\quad$

corresponding to a ratio charge/mass for H⁺, further wherein the ionsare created by electronic bombardment only in the vicinity of a planeparallel to {right arrow over (B)}₁, making an angle of 45° with {rightarrow over (E)}₁, and a heated filament F for emitting electrons beingstretched above the electrodes a₁,a₂, along a line where x=y, theelectron beam being limited by a flat rectangular diaphragm parallel toF and located between F and the electrodes a₁,a₂, such that a resultingselected ion beam is parallel to the x axis when crossing S₁.
 2. A massspectrometer according to claim 1, wherein at the exit S₁, the ions aresubmitted to a second couple of crossed fields {right arrow over(E)}₂,{right arrow over (B)}₁, {right arrow over (E)}₂ being createdwithin a second set of plane parallel electrodes b₁, . . . , b_(i), . .. b_(n), also fitted with properly located slits for the ionstransmission, a value of the electric field E₂ being equal to E₁ cos 45°and a direction of the electric field E₂ making an angle of 45° with E₁,a second selection slit S₂ being located on the cycloid path on theelectrode b_(n) at a point defined by coordinates X≅8.9d and Y=0 in afurther reference system defined by axis X and Y, wherein the X axis andthe Y axis are respectively perpendicular and parallel to E₂and anorigin of the further reference system is fixed at S₁.
 3. A massspectrometer according to claim 1, wherein after the selection slit S₁,the ions are accelerated by an electrostatic field {right arrow over(E)}₂, and introduced in a narrow gap of a magnetic circuit M₂ where themagnetic induction B₂ is perpendicular to B₁ and so are the magneticcircuits M₂ and M₁, a final selecting hole being located at a pointhaving coordinates X=Y=R₂, R₂ being a radius of curvature for the ionstrajectories corresponding to the selected number of mass n.
 4. A massspectrometer according to anyone of claims 1 to 3, wherein the ion beamintensity is modulated by a grid located on the ions trajectory, andpolarized at a fixed frequency, the useful signal being then detectedindependently of the continuous noise.